The given circuit arrangements produce undesired radio interference, among other things, during operation, if suitable countermeasures are not present. Causes for this are for the most part rapid changes in potential--referred to in the following as HF potential for brevity--on the printed circuit board, e.g., by rapid switching transistors, relative to the grounded housing (devices of safety class I) or relative to the environment or ground (devices of safety class II). Electrical HF fields associated with rapid changes in potential can influence, by capacitive couplings, in-phase interference currents, which flow, for example, on the mains input lines. The current loop is thus essentially closed between the circuit arrangement and ground via parasitic capacities. An in-depth description of the formation of radio interferences is found, for example, in W. Hirschmann and A. Hauenstein: "Switching network components", Siemens AG, Berlin, 1990, pp. 72 ff. With respect to the limiting values for radio interference, particularly in the case of electrical operating devices for lamps, VDE 0875, which corresponds to the international standard CISPR 15, must be upheld.
A common measure for suppressing in-phase interference currents consists of connecting an anti-interference filter, e.g., a current-compensated choke in the mains input lines of the circuit arrangement. The design of current-compensated chokes, is explained, for example, in O. Kilgenstein: "Switching network components in practice", Vogel Buchverlag, Wurzburg, 1986, pp. 355 ff. Its effect is based on the fact that the actual current of the mains frequency can pass unattenuated. In-phase interference currents of high frequency, on the other hand, are filtered out by the high inductivity of the current-compensated choke. Of course, limits are placed on a compact construction, since the anti-interfering effect of a current-compensated choke is reduced or even can be reversed into an opposite effect, particularly due to magnetic interference fields, by directly adjacent component parts and their interference signals.
In devices of safety class I, Y capacitors can also be connected from the mains input line toward safety or ground conductors, whereby at least part of the in-phase interference currents can flow out to ground. In apparatuses of safety class II, this is not possible.
EP Patent 0 264 765 describes an electronic converter for the operation of low-volt halogen bulbs, which has a current-compensated choke for radio interference. In addition, the secondary side of the power transformer--this also serves as a decoupling circuit, which transforms the phased voltage of the switching part to the nominal voltage of the connected low-volt halogen bulb--is also connected to the plus or minus pole of the mains rectifier via a capacitor. In this way, an HF short circuit is produced, which minimizes interference voltages by means of the power transformer. Of course, this measure is limited to electronic converters.
In DE 41 37 207 A1, an HF-anti-interference is disclosed, which is also based on an HF short circuit and can be utilized in principle both for ELB's as well as in electronic converters. For this purpose, an HF signal, for example, in the case of an ELB, is decoupled from the serial resonance circuit of the discharge lamp and is connected with an anti-interference choke connected in the mains input line via a high-pass filter. With an optimal dimensioning of the high-pass filter, almost no interference currents flow over the mains input lines. Of course, the HF impedance of the anti-interference choke varies as a function of the value of the input voltage flowing through it. In this way, the anti-interference effect is sensitively modified with the connected load.